1. Field of the Invention
The present invention relates to a tyre for vehicle wheels.
In more detail, the invention concerns a tyre of the winter type for high performance and ultra high performance cars, i.e. cars provided with particular performance qualities and generally having a rear-wheel drive.
2. Description of the Related Art
By winter tyre it is intended a tyre provided with a tread band suitable for running on surfaces of reduced compactness in particular snow-covered roadways.
Tyres having the above qualities are usually required to possess, together with optimal features in terms of traction power, braking and handling on a snow-covered roadway, a good behaviour on dry and wet roads and a satisfactory resistance to wear. Noiseless running also helps in elevating or worsening the qualitative evaluation of a winter tyre.
Usually, the above mentioned behavioural and operational features are determined through formation in the tread band of appropriate circumferential and transverse grooves suitably sized and oriented, which grooves give rise to creation of blocks usually aligned in rows disposed consecutively in side by side relationship and extending circumferentially of the tyre itself.
It addition, of decisive importance as regards the behavioural running features of a car on a snow-covered roadway is the presence of an appropriate lamelliform arrangement of cuts in the blocks, i.e. a thick series of narrow cuts disposed consecutively in side by side relationship in a circumferential direction and oriented substantially transversely with respect to the rolling direction. The task of these narrow cuts, which are currently referred to as sipes, is substantially that of effectively collecting and retaining the snow, since friction of snow against snow is known to be greater than rubber-on-snow friction.
In the international patent application WO 02/068222 in the name of the same Applicant it is described a winter tyre for vehicle wheels comprising a tread band provided with three circumferential grooves and a plurality of transverse grooves that together delimit four circumferential rows of blocks: two axially external shoulder rows and two central rows disposed to the sides of the equatorial plane of the tyre. The transverse grooves converge to the equatorial plane towards a predetermined tyre-rolling direction. On the tread band, each transverse groove belonging to the central rows is provided with an enlarged area in its section having an essentially circular profile, the function of which is to entrap snow. In addition, for obtaining rolling with less noise on dry roads as well, each transverse edge of the blocks comprises at least two successive curvilinear portions. These curvilinear portions are differently shaped and have opposite bends in the two portions to mitigate noise generated by the impact of the blocks when the tyre is rolling on the ground.
The Applicant has perceived the increasing requirement of ensuring more handling on snow-covered roadways to all motor-vehicles and above all to those of the above mentioned high performance type, while at the same time affording more safety and comfort on dry and wet roads.
In fact many automobile houses during their winter tests have begun to regularly carry out behavioural tests on snow, beside the traditional acceleration and braking tests.
These tests are subjective behavioural tests and consist in the tyre being run on a mixed roadway, characterised by straight stretches and bends to be taken at different speeds, as well as by uphill and downhill portions, on the basis of which a test driver gives his/her opinion on different car handling parameters.